1. Field of the Invention
The invention relates to a method of dividing a track on a rotatable magnetizable information storage disc into m sectors by writing information bytes into said track until said track becomes full and in such manner that m groups of said bytes, each containing n bytes and comprising sector identification information, are written in succession with N further bytes written between each said group and the next to be written (if present). The invention also relates to apparatus for carrying out such a method.
2. Description of the Prior Art
It is necessary to subdivide the tracks on a storage disc into sectors, each provided with identification information, in order to facilitate the accessing of a particular set of data in a given track. Accurate subdivision of a track on a storage disc into sectors is limited only by the properties of the disc drive and the read-write channel which accesses the track. Various standard formats have become available, especially for "flexible discs", and the commonest of these in use is written in standard "ECMA-54". When this format is used the part of a sector other than the sector identification information consists of the nominal data field plus a preceding and a succeeding portion which are effectively gaps in the useful information. These gaps are desirable because, when a new set of data is to be written into the data field of a given sector, first of all the times of sector identification information are read from the relevant track until the desired sector number is found. The accessing system is then switched over to the write mode. This switching can only take place a finite time after the end of the relevant sector identification information has been accessed if the latter is not to be destroyed by the newly switched-on erase current. After the new set of data has been written, the erase current has to be switched off, and this must be done in such manner the the erase current is present while the whole of the portion of the track in which the new data set is written in accessed, but is absent when the immediately following sector identification information is accessed. Allowance must also be made for the fact that the rotational speed of the storage disc may deviate from the nominal value, and there are other mechanical and electrical tolerances to be taken into account, so that the gaps take up a significant area of the storage disc. The lengths of the individual portions of the track, i.e. the portions containing the sector identification information and the portions corresponding to the data fields and the gaps are in each case defined by initially filling the track by a given series of bytes, the data content of which bytes is of a predetermined kind where the relevant track portions are to contain, for example, sector identification information or other information of a useful nature, but elsewhere arbitrary. This series is written into each track of the disc by means of an automatic machine specially constructed for this purpose. Between the gap following the data field of the last sector and the sector identification information of the first sector there is a larger gap which also contains a predetermined number of bytes and, in order to achieve this, such automatic machines contain expensive drive mechanisms capable of a very precisely controlled speed. This gap (in the useful information) between the last and the first sectors is chosen to be of such a size that, if the sector subdivision information is subsequently rewritten using a normal commercial drive mechanism, even at the fastest rate of rotation of the disc possible within the tolerance imposed thereon, the end of the last sector will not extend into the beginning of the first sector.
During their useful life, however, such storage discs are only in fact likely to be provided with the sector subdivision information once, i.e. by the manufacturer, the user hardly ever requiring to change it subsequently. Because of this, the gap between the last and the first sectors, which gap is not used for useful storage purposes, is normally superfluous. In other words, this gap (and also the other gaps) are provided to allow for a large number of individual tolerances, whose occurrence is improbable and in practice hardly ever occur at the same time.